band theory of polymers

8/18/2019 Band Theory of Polymers

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Int. J. Electrochem. Sci., 7 (2012) 11859 - 11875

International Journal of

ELECTROCHEMICAL

SCIENCE www.electrochemsci.org

Review Paper

Electronics of Conjugated Polymers (I): Polyaniline

Kerileng M. Molapo, Peter M. Ndangili, Rachel F. Ajayi, Gcineka Mbambisa, Stephen M. Mailu,

Njagi Njomo, Milua Masikini, Priscilla Baker and Emmanuel I. Iwuoha*

SensorLab, Chemistry Department University of the Western Cape, Cape Town ,South Africa*E-mail: [email protected]

Received: 1 October 2012 / Accepted: 3 November 2012 / Published: 1 December 2012

Conducting polymers have elicited much interest among researchers because of their reasonably good

conductivity, stability, ease of preparation, affordability and redox properties compared to other

organic compounds. In particular, the electronic and electrochemical properties of conducting

polymers have made them find applications in photovoltaic cells, organic light emitting diode and

sensors. Among the conducting polymers, polyaniline has received much attention and intensive

research work has been performed with the polymer in its native state or functionalized form. This is

mainly due to the fact that polyaniline and its derivatives or composites or co-polymers with othermaterials are easy to synthesise chemically or electrochemically by oxidative polymerisation. The

mechanism for the synthesis of polyaniline and its electronic properties are presented in this short

review.

Keywords: Band gap, Conducting polymers, Polymerisation, Polyaniline, Semiconductors

1. INTRODUCTION

Traditionally polymers were seen as good electrical insulators and most of their applications

had relied on their insulating properties[1]. However, until three decades ago researchers showed that

certain class of polymers exhibits semiconducting properties[2]. Early studies showed that

polypyrroles exhibit signs of conductivity[3]. This discovery was followed by the work of Shirakawa

and co-workers in 1977 who reported the first synthesis of doped polyacetylene which was formed

accidentally[4]. This incident took place when one of the co-workers mistakenly added excessive

amount of catalyst in the reaction vessel for the polymerisation of acetylene .This resulted in the

formation of a silver film instead of the expected black powder[4]. The new product had different

optical properties compared to the normal black powder; hence by using iodine vapour they attempted

to oxidize polyacetylene in order to obtain its normal optical properties. However, that only resulted in

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Int. J. Electrochem. Sci., Vol. 7, 2012 11860

an increase in the conductivity of the polymer[4]. The discovery along with the extensive research in

this area, contributed to authors (MacDiarmid, Heeger, and Shirakawa[5]) being awarded the Nobel

Prize in Chemistry for the year 2000. Since this discovery reported by Shirakawa and co-workers,

conducting polymers (CPs) have received much attention in the field of science. The aim of this review

is to discuss the role of band gap and molecular orbital theories in determining the electroniccharacteristics of conducting polymers, with particular emphasis on polyaniline.

2. CONDUCTING POLYMERS

A conducting polymer is an organic based polymer that can act as a semiconductor or a

conductor. The most widely studied organic polymers are polyaniline (PANI), polypyrroles,

polythiophenes, and polyphenylene vinylenes[5-6] as given in Fig.1. They are conjugated, that is, they

have π electron delocalisation along their polymer backbone, hence giving them unique optical and

electrical properties [5-7]

**

n

Poly (p-phenylene)

*n

Polyphenylene vinylene

**n

Polythiophene

S

**n

Polypyrrole

NH

HN*

n

Polyaniline

*

**n

Polyisothianaphthene

S

Figure 1. Typical (shown uncharged) structures of conducting polymers


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2.1. Electronics of electrically conducting polymers (ECPs)

Conducting polymers are extensively conjugated molecules: they have alternating single and

double bonds. In these molecules, electrons are able to move from one end of the polymer to the other

through the extended p-orbital system[7]. Hence CPs are known to be either semiconductors or

conductors, which are related to how bands and shells of electrons form within a compound. In view of

the electronics of ECPs, the Band theory is employed to explain the mechanisms of conduction in CPs.

The theory originates from the formation of energy bands in polymer materials from discrete orbital

energy levels found in single atom systems. In this regard, it is vital to review band theory.

2.1.1. The band theory as a function of application of quantum theory

The physical chemistry approach to explanation of band theory is to relate it to the quantum

theory of atomic structures. The first major success of quantum theory was its explanation of atomic

spectra, particularly that of the simplest atom, hydrogen[8]. Quantum mechanics introduced an

important concept which explained that atoms could only occupy well-defined energy states and for

isolated atoms the energy states were very sharp [8]. The spectral emission lines which resulted

correlated to electrons jumping from one allowed energy state to another, and this gave rise to

correspondingly narrow line widths[8]. In a crystalline solid, atoms cannot be viewed as separate

entities, because they are in close proximity with one another, and are chemically bonded to their

nearest neighbour [9]. This leads to the concept that an electron on an atom sees the electric field due

to electrons on other atoms and the nature of the chemical bond implies that electrons on close-

neighbour atoms are able to exchange with one another, causing the broadening of sharp atomic energy

states into energy ‘bands’ in the solid [9]. This can be illustrated using an example below in Fig 2, that

depicts 3p and 3s electron shells for a single metallic atom in the third period of the periodic table that

overlap to become bands that overlap in energy (Atkins, 2002)[9]. The association of these bands is no

longer solely with single atoms but rather with crystal as a whole. In other words, electrons may appear

with equal probability on atoms anywhere else in the crystal.

Figure 2. The formation of bands in a conducting solid in the 3rd period and overlap between the

valence and conduction bands. Reprinted with permission from Reference [9].


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2.1.2. The band theory as a function of application of molecular orbital theory

The chemical approach to band theory is to relate it to molecular orbital theory. In molecular

orbital theory, using H(1) and H(2) hydrogen atoms as an example (Fig.3), an atomic molecular orbital

from H(1)

atom can overlap with an atomic molecular orbital of H(2)

. atom, resulting in the formation of

two molecular orbitals known as the bonding and antibonding molecular orbitals[9]. These are

delocalized over both atoms, and the bonding molecular orbital possess lower energy than the H (1) and

H(2) atomic orbital, while the antibonding molecular orbital has higher energy.

Figure 3. Molecular orbitals in a diatomic molecule.

The energy band that results from the bonding orbitals of a molecule is known as the valence

band, while the conduction band is as a result of the antibonding orbitals of the molecule as illustrated

in Fig. 4. The width of individual bands across the range of energy levels is called band width. The

valence band (VB) represents the highest occupied molecular orbital (HOMO) and the conduction

band (CB) represents the lowest unoccupied molecular orbital (LUMO)[10]. The gap between the

highest filled energy level and lowest unfilled energy level is called band gap (Eg). This band gap

represents a range of energies which is not available to electrons, and this gap is known variously as

‘the fundamental energy gap’, the ‘band gap’, the ‘energy gap’, or the ‘forbidden gap[8] The level of

electrons in a system which is reached at absolute zero is called the Fermi level (F g)[11]. It has been

demonstrated that in order to allow the formation of delocalized electronic states, CPs molecular

arrangement must be conjugated[2]. The delocalization of the electronic states relies on the resonance

stabilized structure of the polymer. The size of the energy band gap depends on the extend ofdelocalization and the alternation of double and single bonds[10]. Moreover the size of the energy


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band gap will determine whether the CP is metal, semiconductor or insulator.[10] Combining the

concepts explained in both atomic and molecular orbital theory, the electronic properties of metals,

semiconductors, and insulators can be differentiated with reference to the energy band gap as shown in

Fig. 4 below.

Figure 4. Energy band diagram demonstrating band gaps

2.1.3. The electronic properties of metals, semiconductors, and insulators

In metals there is no range of energies which is deemed unavailable to electrons, which simply

means that forbidden gap or band gap in metals is Eg = 0 eV[12]. Hence metals always have a partially

filled free-electron band, because the conduction and valence bands overlap. Hence the electron can

readily occupy the conduction band[9]. Insulators have a band gap which is larger than 3 eV (Fig.

4)[12], the energy gap between VB and CB is too large, hence the electron is not able to make that

jump to detach from its atom, in order to be promoted to the valance band. Consequently they are poor

electrical conductors at ambient temperatures. Insulators can be defined as materials in which the

valence bands are filled and the forbidden energy gap between valence band and conduction band is

too great for the valence electrons to jump at normal temperatures from VB to the CB[9].

2.1.4. The electronic properties of semiconductors

There are two possible scenarios which can allow a material to become a conductor. The first

instance is when the VB is not completely filled, hence an electron can raise its energy to a higher

level within the valence band, so that it can detach from its atom[13]. This is coined as conduction

within same band, and it requires a small amount of energy, hence many electrons are capable of

accomplishing it. Second instance is when the band gap is very small, i.e. Eg = 0.1 - 3 eV, hence

electrons can raise their energy and detach from their atoms by jumping to a higher energy level in the

CB [13]. There are several methods, such as thermal or photochemical excitation, which can be used to

excite electrons into the conduction band. Once these electrons have reached the conduction band, they

contribute to the electrical conductivity[9]. Furthermore, the holes which are created in the valence band, by the electrons jumping to the conduction band, also contribute to electrical conductivity, i.e.


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the transfer of charge (electrons or holes), through a polymers backbone causes conduction. Hence

electrical properties of intrinsic semiconductors are determined by the number of electrons in

conduction band and the number of holes in valence band[13]. However, another method known as

doping is also used to generate the charge carriers (electrons and holes). As previously explained

conjugation and the extent of delocalization in CPs determine the size of the energy band gap[10].Generally the more conjugated the CP, the larger the energy band gap, hence most CP’s are unstable in

their undoped state[10]. Doping is a method that involves adding a different element into the material,

and whether a semiconductor is pure (no different element added to it) or it is doped allows

semiconductors to be categorized into either intrinsic or extrinsic semiconductors[12-13].

2.1.4.1. The electronic properties of intrinsic semiconductors

Undoped semiconductors such as pure silicon, is an example of an intrinsic semiconductor. It

possesses a relatively small forbidden energy gap (Eg). Hence because its Eg is small, at 300 K (i.e.

room temperature) the electrons can be thermally excited, causing them to be raised to higher energy in

the conduction band, yielding about 1015-1020 conduction electrons[10]. Fig. 5 demonstrates the

concept of intrinsic semiconductors. As the temperature is increased, more electrons are thermally

excited to the conductions bands and leaving more holes in the valence band.

Figure 5. Band structure diagram of an intrinsic semiconductor[12] (Reprinted with permission)

2.1.4.2. The electronic properties of extrinsic semiconductors

Semiconductor materials into which another element or an impurity is introduced are known as

extrinsic semiconductors. The impurity is known as the dopant, and the process of adding it is referred

to as doping. Dopants are categorised into two groups, i.e. they can either be donors or acceptor[10].

Examples of elements which are donors are phosphorus, arsenic and antimony and typical acceptor


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elements in extrinsic semiconductors are trivalent elements such as boron, indium, gallium and

aluminium[12]. Furthermore, in an extrinsic semiconductor there are two types of conduction

mechanisms. The first mechanism is when an acceptor element is doped with a donor element that has

a higher valence number than it [9]. For example, germanium atom has 32 electrons distributed in four

orbits. The outermost orbital is called the valence orbital, and it contains four electrons, hence it istetravalent. If Ge is doped with phosphorous, which has a valence orbital which contains five electrons

(pentavelent), this creates an extra electron per atom[12]. This extra pair of electrons from donor

phosphorus partially fills the conduction band because it occupies discrete levels near the bottom of the

conduction band, around -0.1 eV of the band gap[12] At room temperature (300 K) the electrons are

excited into the conduction band. This type of conduction is called n-type semiconductivity and uses

electrons as the main charge carriers [10] as illustrated in Fig.6 below.

Figure 6. Band structure diagram of an extrinsic n-type semiconductor

In the second mechanism, a dopant with a lower valency than the material that is being doped

is used, for example a trivalent dopant, such as indium, can be used to dope germanium. In this

scenario, only three covalent bonds are formed from the four possible bonds which could form. Here,

the valence band is now depleted of some electrons because the fourth possible bond is vacant. Thisvacancy creates a hole for each electron missing from a bonding pair[12]. The main charge carriers in

this instance are the holes that are formed when the electrons move out of the valence shell. A trivalent

dopant is characterized as an acceptor because it injects many holes which are the majority carriers of

the charge. These holes are responsible for the conductivity of crystals. For this reason the materials

are called p-type semiconductors[9-10, 12-13]

The electronic properties of CPs and the general band theory described above provide the

insight that can direct the process of designing and synthesising conducting polymers in order to

achieve the desired electronic properties. Polyaniline, polypyrroles, polythiophenes and polyphenylene

vinylenes[6-7] are among the most widely studied materials in the family of conducting polymers.

However, PANI is probably the most important industrial conducting polymer[6] due to the fact that it


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can be processed from solutions into thin film[14], exhibits high environmental stability, has low

synthetic cost[15] and the fact that its morphological, electric and optical properties can be controlled

by doping and deprotonation[16]. These unique properties of PANI necessitated its applications in

areas such as energy storage (light weight batteries, capacitors)[17], energy conversion (photovoltaic

cells), optical signal processing (light emitting diodes)[2, 17-18] and electrocatlysis.

2.2. Polyaniline

Polyaniline was initially discovered in 1834 by Runge, and it was referred to as aniline

black[19]. Following this, Letheby carried out research to analyse this material in 1862 [19]. PANI is

known as a mixed oxidation state polymer composed of reduced benzoid units and oxidized quinoid

units[15]. Green and Woodhead (1912)[20] discovered this interesting characteristic of polyaniline.

Furthermore, it was discovered that PANI had characteristics of switching between a conductor and an

insulator under certain experimental conditions[1]. Since then, the material has become a subject of

great interest in research[21].

2.2.1. Structure of Polyaniline

As a mixed oxidation state polymer consisting of benzoid and oxidized quinoid units[15, 22],

PANI’s average oxidation state is denoted as 1-y whereby the value of y determines the existence of

each of the three distinct PANI oxidation states[15, 20] as shown in Fig. 7. Thus PANI exists as fully

reduced leucoemeraldine (LE) where 1-y = 0, half oxidized emeraldine base (EB) where 1-y = 0.5 andfully oxidized pernigraniline (PE) where 1-y = 1[15]. The EB is regarded as the most useful form of

polyaniline due to its high stability at room temperature, it is composed of two benzoid units and one

quinoid unit that alternate and it is known to be a semiconductor[23].

N

H

N

H

N N

ny 1-y

Figure 7. Different oxidation states of polyaniline (y = 1: leucoemeraldine, y = 0.5: emeraldine and y

= 0: pernigraniline

Furthermore, EB can be doped in a non-redox reaction in acidic medium which results in an

emaraldine salt (ES). On the other hand LE is easily oxidized while the PE is easily degraded[15, 23].


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2.2.2. Synthesis of polyaniline

There are several methods that can be employed to synthesize conducting polymers; such as

electrochemical oxidation of the monomers and chemical synthesis[24]. Moreover uncommon methods

such as enzyme-catalyzed polymerisation and photochemically-initiated polymerisation are also

employed[25] in some instances. Essentially during polymerisation, monomers are used as starting

materials to form low molecular weight oligomers. At potentials that are lower than that at which the

monomers are oxidized, the low molecular weight oligomers are further oxidized to form a

polymer[24]. In the case of electrochemical polymerisation, the monomers have to be oxidized by

cycling between a potential window which will allow for the oxidation to take place and the polymer

will then be electrodeposited onto the substrate[24-25]. In the case of chemical synthesis chemical

oxidants such as ferric chloride (FeCl3) and ammonium persulfate [(NH4)2S2O8] are employed and the

polymer precipitates out of the chemical reaction solution[24]. The current review focuses on

electrochemical and chemical polymerisation and these will be further discussed.

2.2.3. Electrochemical polymerisation

Electrochemical polymerisation is commonly carried out by employing one of three techniques.

These methods involve the application of (i) a constant voltage (potentiostatic), (ii) a variable current

and voltage (potentiodynamic) and (iii) a constant current (galvanostatic) to an aqueous solution of

aniline[26]. To carry out electropolymerisation by using any of the three mentioned techniques, a three

electrode assembly is required. These electrodes are known as counter electrode, reference electrode

e.g. Ag/Ag chloride and a working electrode. There electrosynthesized polymer is deposited onto the

working electrode which can be a platinum gold, glassy carbon or indium tin oxide (ITO). An

electrolyte such as an acid (HA) is also needed for polymerisation[27]. It has several functions such as,

to provide a sufficiently low pH so as to help solubilize the aniline monomer in water and to avoid

excessive branching of undesired products, but instead to generate doped emeraldine salt (ES) form

[27].

2.2.4. Chemical polymerisation

For chemical polymerisation to take place monomers have to be oxidized to initiate the

reaction. (NH4)2S2O8 which has oxidation potential (E0 = 1.94 V) and FeCl3 with E0 = 0.77 V are the

most commonly used oxidizing agent[28]. However, there are other less commonly used oxidants such

as hydrogen peroxide [(H2O2); E0 = 1.78 V], cerium (IV) sulfate [Ce(SO4)2; E0 = 1.72 V] and

potassium dichromate [K 2Cr 2O7; E0 = 1.23V][26]. Ferric chloride has the lowest oxidation potential

compared to the other four oxidizing agents but it has proved to be a useful oxidizing agent yielding up

to 200000 molecular weight polyaniline chain[20]. As is the case with electrochemical polymerisation,

acidic pH conditions (pH < 3) are required for a chemical polymerisation. The low acidic pH

minimises the formation of undesired branched product[29] .


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2.2.5. Mechanism of polymerisation

The chemical or electrochemical polymerisation of aniline proceeds via a radical propagation

mechanism as shown in Schemes 1-6. The initial steps (1 and 2) are common to both methods, but

subtle differences appear in the initial product of the chain propagation step and product formation

steps (3 and 4)[30]. Accordingly steps 3 and 4 will be illustrated separately for chemical and

electrochemical polymerisation[25].

Step 1: Oxidation of Monomer

In the initial step, as shown in Scheme 1, the oxidation of aniline to a radical cation, which

exists in three resonance forms, takes place. This step is the slowest step in the reaction, hence it’s

deemed as the rate determining step[30] in aniline polymerisation.

NH2HA

-e-

N

H

H.+

A-

NH2

.

+

A-

NH2+

A-

.

Scheme 1. Oxidation of monomer during electrochemical polymerisation of aniline.

Step 2: Radical coupling and re-aromatisation

Head to tail coupling of the N - and para- radical cations (Scheme 2) takes place, yielding a

dicationic dimer species[31]. This dimer further undergoes the process of re-aromatisation which

causes it to revert to its neutral state, yielding an intermediate referred to as p-aminodiphenylamine

(PADPA). These processes are also accompanied by the elimination of two protons[30].

NH2N

H

H.+

A-

NH2+

A-

.

NH2+

A-

+

A-

NH NH2

-HA

Scheme 2. Radical coupling and re-aromatization during electrochemical polymerisation of

aniline[30].

Step 3 : Chain propagation for electrochemical synthesis

The dimer radical cation (centred on the para position; nitrogen atom) undergoes oxidation on

the surface of the electrode[30]. Chain propagation results from the radical cation of the dimercoupling with an aniline cation as illustrated in Scheme 3.


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NH2+

A-

.

A-

NH NH2NH NH2-e

-

HA

+ .

A-

NH NH2+ .

NH NH NH2

Scheme 3. Chain propagation during electrochemical polymerisation of aniline.

Step 4 : Oxidation and doping of the polymer for electrochemical synthesis

Scheme 4 shows the oxidation and doping of the polymer during electrochemical

polymerisation reaction in acid solution which dopes the polymer to yield PANI/HA[31].

A-

NH NH

NH NH-e

-

HA

+.

n

n

Scheme 4. Oxidation and doping of the polymer during electrochemical polymerisation of aniline.

Step 3 : Chain propagation for chemical synthesis

In the chain propagation during the chemical synthesis of polyaniline, the initial product is the

fully oxidized pernigraniline salt as shown in Scheme 5 below.

A-

NH NH2

NH NH2

HA

+.

A-

A-

+. +.

N

A-

+.

H

H

A-

NH+. +.

A-

Scheme 5. Chain propagation of the polymer during chemical polymerisation of aniline.

Step 4: Reduction of the pernigraniline salt to emeraldine salt

When all the oxidant in the reaction mixture is completely used up, the pernigraniline salt

formed in step 3 of the chemical synthesis mechanism is reduced by unreacted aniline to the green

emeraldine salt (Scheme 6).


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A

NH

NH NH

+.

A A

+. +.

HN

n

NH2

NH2+.

+

n

Scheme 6. Reduction of the pernigraniline salt to emeraldine salt during chemical polymerisation of

aniline[26].

2.2.6. Properties of polyaniline

It is well known that polyaniline has switching, optical, conductive and solubility propertiesthat distinguish it from other conducting polymers (Wallace et al ., 2002) [22]. These properties of

aniline will be discussed in this section.

2.2.6.1. Switching and Optical properties of PANI

Figure 8. Cyclic voltammogram of polyaniline film on a platinum electrode in 1 M HCl at a scan rate

0.1 V s-1. (The potentials at which structure and colour changes occur and the change in the

potential of the second redox reaction with pH are shown as well).

The switching and optical properties of PANI are interlinked and influence each other directly.

The tendency of PANI to switch between oxidation states directly influences it’s UV -vis absorption

characteristics[26], and as a result the two properties will be discussed concurrently. Once theconductive form of the PANI is formed, i.e. emaraldine salt, the addition of an acid or a base that


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protonates and deprotonates the base (-NH-) sites in PANI causes switching of PANI between

oxidation states [32].

This means that there is a pH dependence of the oxidation states of PANI. When the pH of the

polymerisation reaction is greater than four PANI becomes non-electroactive. The loss of its

electroactivity is due to the failure of the formation of the ES salt [32]. PANI is a mixed oxidation state polymer, ranging from the most reduced leucoemerladine form which is yellow in colour, to the half

oxidized emerladine which is green, and the violet fully oxidized pernigraniline[33] as illustrated in

Fig. 8. The electrochemical switching of polyaniline between oxidation states can be readily monitored

by cyclic voltammetry as illustrated in Fig. 8. The voltammogram shows the trend of structural

switching of PANI with pH and redox potentials. Furthermore the UV-visible spectrum of polyaniline

is sensitive to oxidation state and transitions between oxidation states and is accompanied by visible

optical colour change [32]. Hence, absorbance parameters of the polymer can be used to calculate the

electronic band gap of the various PANI structures. The absorbance characteristics are also linked to

the signature transitions which take place during the switching between oxidation states of PANI.

When quantifying the band gap of the different oxidation states with UV-visible data, the electronic

band structures are linked to one of the oxidation states of PANI[34]. The PANI in the ES form

exhibits three characteristic bands, one at 330 nm attributed to π-π* band, and two bands at 430 and

800 nm in visible region which are due to π-polaron band and polaron-π* band transitions[35] as

illustrated in Fig. 9.

Figure 9. The different band gaps present in polyaniline redox states[ 34]. (Reprinted with permission)

The emeraldine base form of PANI shows a low wavelength π-π* band and a strong absorption

band at ca. 600 nm attributed to a local charge transfer between a quinoid ring and the adjacent imine-

phenyl-amine units (intramolecular charge transfer exciton)[25]. The reduced leucoemeraldine base


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exhibits a band at ca. 320 nm attributed to the π-π* electronic transition[34]. Pernigraniline base shows

two bands at ca. 320 nm (π-π* band) and at ca. 530 nm (Peierl gap transition)[25].

2.2.6.2. Conductivity properties of PANI

It has been previously demonstrated that the polyaniline chain can be formed by various

combinations of the two repeating units known as the X and Y components of polyaniline[34]. Owing

to this, PANI has many unique properties and electronic conduction mechanisms that distinguish it

from the rest of the conducting polymers. For example the conductivity of PANI varies with the extent

of oxidation (variation in the number of electrons) and the degree of protonation (variation in the

number of protons).

N N NH NH

Polyemeraldine

H+ A

- Protonation

HN

HN NH NH

Bipolaron form

A- + A

- +

Dissociation of bipolaron to form two polarons

H

N

H

N NH NH

Polaron form

A- +

A-+. .

Delocalization of polarons

HN

HN NH NH

A- +

A-+. .

Resonance forms of delocalized polaron lattice

H

N

H

N NH NH A

- + A

-+. .

Scheme 7. The doping of EB with protons to form the conducting emeraldine salt (PANI/HA) form of

polyaniline (a polaron lattice)[26] (Reprinted with permission)

Among the various oxidation states that PANI can exist in, the one that can be doped to a

highly conductive state is the moderately oxidized emeraldine base[32]. This form of PANI has a

structure which consists of equal proportions of amines ( – NH – ) and imine (=N – ) sites. Through

protonic acid doping, imine sites are protonated by acids HA to the bipolaron (dication salt) form[36].The bipolaron then undergoes a further rearrangement to form the delocalized polaron lattice which is


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a polysemiquinone radical-cation salt as shown in Scheme 7 below. The resulting emeraldine salt has

conductivity on a semiconductor level of the order of 100 S cm – 1, which is many orders of magnitude

higher than that of common polymers (104 S cm –

1)[37]. Only 1% of the charge carriers which are available in the ES salt actually contribute to its

observed conductivity. If all the available charge carriers were to contribute, the resulting conductivityat room temperature would be ~ 105 S cm-1, which is comparable to that of copper[38].

2.2.6.3. Solubility properties of PANI

The emeraldine base, which is a less ionic state of PANI, is soluble in a number of organic

solvents such as, dimethyl sulfoxide, chloroform, tetrahydrofuran, dimethyl formamide, and methyl

pyrrolidinone[39]. This is because in this state there are no cationic charges present in the polymer

backbone. However, the emeraldine salt is insoluble in aqueous solutions and most common organic

solvents because in this state PANI has cationic charges present in its polymer backbone[39]. It has

been demonstrated that by introducing functionalised protonic acid dopants such as sulfonic acids (10-

champhor sulfonic acid and dodecyl-benzene sulfonic acid) into the ES salt, solubility can be improved

although it will be limited[40]. Furthermore, poly(2-methoxyaniline-5-sulfonic acid) (PMAS) has been

reported to give a unique nanoscale water-soluble material containing a conducting PANI backbone

and a conducting PMAS dopant, which exhibits multiple switching capabilities[41]. Another example

of a sulfonated PANI derivative is polyanilino naphthathelene sulfonic acid (PANSA) which is

synthesized from 8-anilino1-naphthalene sulfonic acid monomer. The monomer is viewed as aniline

substituted with naphthalene sulfonic acid. It has been reported in an experiment where PANSA wasused as a platform for preparing an enzyme-based biosensor for the determination of ethambutol[42],

that the presence of naphthalene sulfonic acid aids the solubility of the polymer.

3. CONCLUSION

The propensity of the morphology of polyaniline to be customised by controlling the synthesis

conditions has resulted in the production of polyaniline of various structural descriptions including

nano-tubes, nano-rods and nanoparticles. This structural changes that are realisable with polyanilineare made possible by the use of dopants such has high molecular weight sulfonic acids. Accordingly,

the variations in the molecular aggregation of the polymer mean that a range of band gaps, electronic

and electrical properties are realisable with doped polyaniline. This, therefore, makes it possible for the

polymer to have a wide application in electrochemistry, sensors and electrochemical energy systems.

ACKNOWLEDGEMENT

The authors would like to acknowledge the financial support from South African Synthetic Oil Limited

(SASOL) and National Research Foundation of South Africa (NRF).


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